1. Field of the Invention
This invention relates to an ion source, and more particularly to improvements in a microwave discharge ion source which is suited to, for example, an ion implanter for implanting ions into a semiconductor wafer.
2. Description of the Prior Art
A microwave discharge ion source has the great features that the lifetime is long and that ion beams of high currents can be produced. It is therefore used as an ion source for an ion implanter. The microwave discharge ion source is described in detail in the specification of U.S. Pat. No. 4,058,748 issued Nov. 15, 1977.
FIG. 1 shows the fundamental construction of the prior-art microwave discharge ion source. Referring to the figure, microwaves generated by a microwave generator 1 are propagated through a rectangular waveguide 2 and are introduced into a discharge chamber 5 via a rectangular waveguide 4 having ridged electrodes 3 and 3. The discharge chamber 5 is vacuum-sealed from the side of the rectangular waveguide 4 by a vacuum-sealing dielectric plate 6. The discharge chamber 5 is constructed of ridged electrodes 7 and 7, a discharge space 8 formed between the ridged electrodes 7 and 7, a conduit (not shown) for introducing a gas to be ionized, and a dielectric (not shown) packed in a space other than the discharge space 8. The microwaves introduced into the discharge chamber 5 generate an intense microwave electric field between the ridged electrodes 7 and 7. Further, an intense magnetic field is applied to the discharge chamber 5 in a direction (in FIG. 1, the axial direction) intersecting orthogonally to the microwave electric field generated between the ridged electrodes 7 and 7. In order to generate this magnetic field, a solenoid 9 is disposed at the outer periphery of the discharge chamber 5. The sample gas to be ionized is introduced into the discharge space 8 by the gas conduit (not shown), and a plasma of high density is produced in the discharge space 8 by the interaction between the microwave electric field and the magnetic field established within the discharge space 8. An ion beam 10 is extracted by ion extraction electrodes 11 from the high-density plasma thus generated.
The extracted ion beam 10 irradiates a sample 40 such as of a semiconductor. A vacuum chamber 14 is maintained in a vacuum state by a vacuum system 15.
As stated above, in the construction of the prior-art microwave discharge ion source, the vacuum-sealing dielectric plate 6 through which the microwaves propagate and which also works as a vacuum sealing for the discharge chamber 5 is disposed between the discharge chamber 5 and the rectangular waveguide 4 having the ridged electrodes 3 and 3. The vacuum-sealing dielectric plate 6 has the two functions as described above, one of which is to have the microwaves propagate through the rectangular waveguide 4 having the ridged electrodes 3 and 3, to the discharge chamber 5 without reflection and the other of which is to keep the interior of the discharge chamber 5 at a vacuum. To the end of fulfilling the first function, the sectional shape of the vacuum-sealing dielectric plate 6 needs to be similar to either the sectional shape of that part of the rectangular waveguide 4 having the ridged electrodes 3 and 3 which lies in contact with the vacuum-sealing dielectric plate 6, as shown in FIG. 2A (section a--a in FIG. 1), or the sectional shape of the discharge chamber 5 lying in contact with the vacuum-sealing dielectric plate 6, as shown in FIG. 2B (section b--b in FIG. 1). That is, the vacuum-sealing dielectric plate 6, in its sectional shape, needs to be: (1) a rectangular plate which has metal parts corresponding to the ridge portions 3 and 3 of the rectangular waveguide 4 having the ridged electrodes shown in FIG. 2A and in which a part corresponding to the other space 12 is filled with a dielectric, or (2) a circular plate 6' as shown in FIG. 3A which has metal parts 7' and 7' corresponding to the ridged electrodes 7 and 7 of the discharge chamber 5 shown in FIG. 2B and in which a part 13' corresponding to the other space 13 is filled with a dielectric. To the end of fulfilling the second function, a dielectric having an excellent high-frequency characteristic and being non-porous, such as forsterite ceramics and aluminous ceramics, is the most suitable as the dielectric material which fills the part 13' corresponding to the aforecited space 13. These materials, however, are sintered dielectrics and are quite unsuitable to be molded by machining. Further, in case where a plate in a complicated shape having corners as in the sectional shape of the dielectric part 13' illustrated by way of example in FIG. 3A is fabricated with the aforecited dielectric material and by sintering, the finish accuracy of the dielectric part 13' is very inferior. In order to enhance the finish accuracy even slightly, the sintering must be repeatedly performed by remaking dies. In addition, in order to achieve the second function of the vacuum-sealing dielectric plate 6 or the vacuum sealing by the use of the circular plate 6' of such structure made of the composite consisting of the metal parts 7' and 7' and the dielectric part 13', the metallization of and welding to the dielectric part 13' are required, the simultaneous machining of the metal parts 7' and 7' and the dielectric part 13', etc., so that the cost of the circular plate 6' is expensive. With the intention of solving these problems, a vacuum-sealing dielectric plate 6" as shown by way of example in FIG. 3B was fabricated in which a dielectric part 13" had a shape substantially corresponding to the shape shown in FIG. 3A. Circular parts 7" and 7" corresponding to the ridged electrode parts 7' and 7' were formed merely by inserting discs which were separately fabricated of copper by machining. The vacuum sealing was effected by means of two O-ring gaskets somewhat larger in diameter than the circular parts 7" and 7" and one O-ring gasket somewhat smaller in diameter than the vacuum-sealing dielectric plate 6". A good result was obtained as to the vacuum sealing, whereas the reflection of the microwaves from the vacuum-sealing dielectric plate 6" came to occur. This will be ascribable to the change of the shape and the mismatching of the impedance attributed to the fact that the ridged electrode parts 7' and 7' in FIG. 3A were turned into the circular parts 7" and 7" as shown in FIG. 3B. Regarding this problem of the mismatched impedance, since the sectional shape of the vacuum-sealing dielectric plate 6" is complicated as shown in FIG. 3B, the calculation of the impedance is very difficult. In case of such sectional shape, accordingly, it is in effect impossible to achieve the matching of the impedance in the vacuum-sealing dielectric plate 6". Therefore, such problems occurred that the reflection of the microwaves from the vacuum-sealing dielectric plate 6" developed, that the loss of the output of the microwave generator 1 increased due to the reflection, and that when a microwave generator 1 of high power was used in order to cover the output loss, abnormal sparks were incurred in the vicinity of the vacuum-sealing dielectric plate 6".